The biomechanical effects of acromial fracture angulation in reverse total shoulder arthroplasty

Background: The biomechanical changes and treatment guidelines on acromial fracture after reverse shoulder arthroplasty (RSA) are still not well understood. The purpose of our study was to analyze the biomechanical changes with respect to acromial fracture angulation in RSA.Methods: RSA was performed on 9 fresh-frozen cadaveric shoulders. An acromial osteotomy was performed on the plane extending from the glenoid surface to simulate an acromion fracture. Four conditions of acromial fracture inferior angulation were evaluated (0 degrees, 10 degrees, 20 degrees, and 30 degrees angulation). The middle deltoid muscle loading origin position was adjusted based on the position of each acromial fracture. The impingement-free angle and capability of the deltoid to produce movement in the abduction and forward flexion planes were measured. The length of the anterior, middle, and posterior deltoid was also analyzed for each acromial fracture angulation.Results: There was no significant difference in the abduction impingement angle between 0 degrees (61.8 degrees +/- 2.9 degrees) and 10 degrees angulation (55.9 degrees +/- 2.8 degrees); however, the abduction impingement angle of 20 degrees (49.3 degrees +/- 2.9 degrees) significantly decreased from 0 degrees and 30 degrees angulation (44.2 degrees +/- 4.6 degrees), and 30 degrees angulation significantly differed from 0 degrees and 10 degrees (P < .01). On forward flexion, 10 degrees (75.6 degrees +/- 2.7 degrees), 20 degrees (67.9 degrees +/- 3.2 degrees), and 30 degrees angulation (59.8 degrees +/- 4.0 degrees) had a significantly decreased impingement-free angle than 0 degrees (84.2 degrees +/- 4.3 degrees; P < .01), and 30 degrees angulation had a significantly decreased impingement-free angle than 10 degrees. On analysis of glenohumeral abduction capability, 0 degrees significantly differed (at 12.5, 15.0, 17.5, and 20.0N) from 20 degrees and 30 degrees. For forward flexion capability, 30 degrees angulation showed a significantly smaller value than 0 degrees (15N vs. 20N). As acromial fracture angulation increased, the middle and posterior deltoid muscles of 10 degrees, 20 degrees, and 30 degrees became shorter than those of 0 degrees; however, no significant change was found in the anterior deltoid length.Conclusions: In acromial fractures at the plane of glenoid surface, 10 degrees inferior angulation of the acromion did not interfere with abduction and abduction capability. However, 20 degrees and 30 degrees of inferior angulation caused prominent impingement in abduction and forward flexion and reduced abduction capability. In addition, there was a significant difference between 20 degrees and 30 degrees, suggesting that not only the location of the acromion fracture after RSA but also the degree of angulation are important factors for shoulder biomechanics.